[Python-ideas] default parameter in fuctions to clean up flow

I have had a long standing unanswered question on on stackoverflow: is it possible to pass a function to a default parameter so that you could do in essence things like this. def time_diff(target_time, curr_time= lambda : datetime.now() ): return curr_time - target_time this would be an syntactical improvement over this style where you have if statement to initialize a missing parameter. def time_diff(target_time, curr_time=None): if curr_time == None: curr_time = datetime.datetime.now() return curr_time - target_time